Machine vision inspection of objects would be generally familiar to one of skill in the art. Systems and methods of machine vision inspection typically use one or more cameras and, frequently, one or more sources of illumination. Illumination may be provided in various forms including, but not limited to, direct illumination, back light illumination, ring illumination, horizontal illumination, coaxial illumination, and dome illumination.
When inspecting an object having a specular (shiny) surface, a super-diffuse illumination environment is particularly effective. Of the aforementioned illumination types, it has been found that dome illumination works very well for inspecting such objects, as well as objects having an irregular (e.g., convex, spherical, semi-spherical) shape. A machine vision inspection device that provides dome illumination is generally referred to as a dome illuminator.
As illustrated in FIG. 1, known dome illuminators typically include a hemispherical dome that overlies an object to be inspected (imaged). An illumination source, such as a ring of LEDs, is located within the dome. The interior surface of the dome is typically painted or otherwise coated with a diffusely reflective material such that light emitted by the illumination source is reflected off the dome and onto the object to be imaged.
Dome illuminators also include a central (axial) aperture that passes through the dome to provide a viewing window for an associated camera. The camera is focused on the upwardly facing surface of the object to be imaged, which surface is illuminated by the light being reflected off of the dome interior. An unfortunate side-effect of this design, however, is that the central aperture appears in reflection as a dark spot on the surface being imaged. This may result, for example, in an inability to read a code imprinted on the surface being imaged or an inability to see other features thereof.
A known solution to this problem is a device commonly referred to as a Cloudy Day® illuminator (CDI). A CDI typically includes a dome illuminator with a second co-axial light source that further illuminates an object to be inspected by projecting light through the central dome aperture. The brightness of the dome and co-axial light sources are balanced until they appear of equal brightness in reflection from the object surface being inspected.
While a CDI is an improvement over a typical dome illuminator, CDIs are expensive. Further, the addition of the co-axial light source adds to the size, and mechanical and electrical complexity of the dome illuminator, and also renders typical CDIs quite bulky. CDIs also employ a flat or curved beam splitter arrangement, which is usually fragile and difficult to clean.
It can be understood from the foregoing commentary that it would be desirable to provide a simple, cost-effective and compact machine vision inspection device that is capable of accurately imaging (inspecting) the entire specular surface of an object. Embodiments of the present invention are such devices.